As is well known, conventional navigation devices compatible with the Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) can aid users of such devices in determining their positions relative to various navigation satellites configured in a constellation. In this regard, satellite trajectory information is generally broadcast to such conventional navigation devices (e.g., GPS receivers), by satellites as ephemeris data from which satellite positions can be predicted. These devices then use the satellite ephemeris position information along with ranging measurements to solve for their own positions using a triangulation process or similar process.
However, due to gravity perturbation effects (e.g., caused by the Earth gravity harmonics, sun, moon, and other bodies) and other external perturbations (e.g., solar pressure disturbances), such ephemeris data is generally accurate for only short periods of time. Indeed, predicted satellite positions determined using such ephemeris data may deviate from the true (i.e., actual) satellite positions by as much as a few hundred meters for just a few hours outside of the time interval of validity. Errors of this magnitude are too large for most GNSS based navigation applications, and the trend over the past two decades has been to reduce the Signal-in-Space (SIS) ranging error due to ephemeris and satellite clock from several meters to the current level of one or two meters for GPS satellites. The objective for future GNSS satellites is to further reduce the ranging error to less than one meter.
As a result, satellites may be frequently uploaded (e.g., about once per day) with new ephemeris data sets to be broadcast by the satellites to conventional navigation devices. The nominally proposed solution to reduce errors for future GNSS is to upload new ephemeris data even more frequently, e.g. once per hour, or once every few hours. Each piecewise ephemeris data set typically covers a limited time interval (e.g., about 4 hours) which accounts for known and predictable forces (i.e., forces other than variable solar pressure or other unpredictable small forces), with successive ephemeris data sets that overlap by one or two hours.
Unfortunately, there are significant disadvantages to this approach. For example, due to limited broadcast data rates, each navigation satellite only broadcasts the “current” ephemeris data set, and then switches to a new “current” ephemeris data set broadcast about every two hours. Conventional navigation devices must then typically read the new “current” ephemeris data set about every two hours to maintain full accuracy. In particular, if the conventional navigation devices rely on stale ephemeris data sets, accumulated deviations between the actual satellite positions and the stale ephemeris data can result in significant navigation errors.
In addition, the above approach renders conventional navigation devices susceptible to navigation errors induced by the interruption of satellite ephemeris data broadcasts. Such interruptions may be caused, for example, by signal jamming, signal attenuation, line-of-sight blockage (e.g. urban canyon environments), weak Signal-to-Noise Ratio (SNR) conditions, or other forms of interference. While the navigation device can continue to make ranging measurements at very low signal power levels or with a short span of data of one second or less, strong signals are required to read new ephemeris information over the entire data broadcast interval of up to 30 seconds. In this regard, the navigation device may be configured to integrate the received signal for longer intervals of time to filter noise effects while making ranging measurements (e.g., one second), whereas the integration interval for demodulation of the broadcast ephemeris data is typically limited to 20 milliseconds due to the broadcast data rate of 50 Hz. The ratio of these two intervals (i.e., 50 or 17 dB), is an estimate of the relative weak signal capability advantage associated with making ranging measurements versus demodulating broadcast data.
In addition, once a navigation device is first turned on, it normally must read new ephemeris data from the satellites. The time delay to obtain a solution after the device is turned on is known as Time-to-First Fix (TTFF). The delay of about 30 seconds just to read this data often causes the TTFF to approach one minute for most stand-alone devices. Many users would prefer smaller TTFF of only a few seconds.
As a result, there is a need for an improved approach to satellite-based navigation that does not rely on frequent ephemeris updates to be received by conventional navigation devices. In particular, there is a need for a satellite-based navigation approach that may permit user devices to continue providing accurate position information for long intervals of time to improve TTFF and operational performance despite the possible presence of satellite signal interference, line-of-sight obstructions, signal attenuation, or weak signal conditions.